Interpretive Summary: The manner in which fatty acid units fit together within animal fat may influence the packing density of the fat and rate of utilization of fat by the animal. With a combination of NMR and molecular modeling techniques, we found that storage of more saturated FA and mobilization/oxidation of polyunsaturated FA is energetically favorable from a molecular standpoint. It was found that fat from obese mice was enriched in oleate and denser than normal mouse fat; results also showed more efficient triglyceride packing. These results may explain some of the differences in which normal and obese mice mobilize fat.

Technical Abstract:
A theoretical framework describing fatty acid (FA) relative mobilization requires considering adipose tissue as an extended three-dimensional structure, with packing of triacylglycerols (TG) that globally energy minimizes the adipose network. With a combination of NMR and molecular modeling techniques, we found that storage of more saturated FA and mobilization/oxidation of polyunsaturated FA is energetically favorable from a molecular standpoint. 13C 2d NOSEY NMR chemical exchange data were obtained from normal and obese mouse fat. Cross peaks fat showed that FA associated head to foot. Obese mouse fat was enriched in oleate and denser than normal mouse fat; results showed additional cross peaks consistent with more efficient TG packing. Since -CH3, -CH2CH3 and -CH2CH2C=O each occur on the FA in a TG, the 1D NMR spectrum peak intensities would be expected to be equal and independent of TG sequence. Instead, intensities would be expected to be equal and independent of TG sequence. Instead intensities are greater in obese vs. normal mouse fat at the sites with NMR cross peaks. This indicates that in natural adipose, a concentrated TG solution, the NMR signals which correspond to various functional groups are not "pure TG", but are influenced by polarization transfer from the extended adipose network. Molecular modeling showed that head/foot/head/foot TG tetramers are stable and compact and may be the minimum structural unit in adipose tissue. Within TG structures, typically energy increases with degree of unsaturation, but oleic acid can be associated with lower energy structures than saturated FA, and alpha linolenic acid can be associated with higher energy structures than docosahexanoic acid.